1. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for cancelling echos in telephone systems.
2. Description of the Related Art
Every current land-based telephone is connected to a central office by a two-wire line (called the customer or subscriber loop) which supports transmission in both directions. However, for calls longer than about 35 miles, the two directions of transmission must be segregated onto physically separate wires, resulting in a four-wire line. The device that interfaces the two-wire and four wire segments is called a hybrid. A typical long-distance telephone circuit can be described as being two-wire in the subscriber loop to the local hybrid, four-wire over the long-haul network to the distant hybrid, and then two-wire to the distant speaker.
Although the use of hybrids facilitates long distance speech transmission, impedance mismatches at the hybrid may result in echos. The speech of the speaker A is reflected off the distant hybrid (the hybrid closest to the speaker B) in the telephone network back toward the speaker A, causing the speaker A to hear an annoying echo of his/her own voice. Network echo cancellers are thus used in the land-based telephone network to eliminate echos caused by impedance mismatches at the hybrids and are typically located in the central office along with the hybrid. The echo canceller located closest to speaker A or B is thus used to cancel the echo caused by the hybrid at the other end of the call.
Network echo cancellers, employed in the land-based telephone system, are typically digital devices so as to facilitate digital transmission of the signals. Since the analog speech signals need to be converted to digital form, a codec located at the central office is typically employed. The analog signals provided from telephone A (speaker A) to central office A are passed through hybrid A and are converted to digital form by codec A. The digital signals are then transmitted to central office B where they are provided to codec B for conversion to analog form. The analog signals are then coupled through hybrid B to the telephone B (speaker B). At the hybrid B, an echo of the speaker A's signal is created. This echo is encoded by the codec B and transmitted back to the central office A. At central office A an echo canceller removes the return echo.
In the conventional analog cellular telephone system, echo cancellers are also employed and are typically located at the base station. These echo cancellers operate in a similar fashion to those in the land-based system to remove unwanted echo.
In a digital cellular telephone system for a call between a mobile station and a land-based telephone, the mobile station speaker's speech is digitized using a codec and then compressed using a vocoder, which models the speech into a set of parameters. The vocoded speech is coded and transmitted digitally over the airwaves. The base station receiver decodes the signal and passes it four-wire to the vocoder decoder, which synthesizes a digital speech signal from the transmitted speech parameters. This synthesized speech is passed to the telephone network over a T1 interface, a time-multiplexed group of 24 voice channels. At some point in the network, usually at the central office, the signal is converted back to analog form and passed to the hybrid at the subscriber loop. At this hybrid the signal is converted to two-wire for transmission over the wire-pair toward the land-based subscriber phone.
For reference purposes, in a cellular call between a mobile station and a land-based telephone, the speaker in the mobile station is the far-end talker and the speaker at the land-based telephone is the near-end talker. As in the land-based system, the speech of the far-end talker is reflected off the distant hybrid in the telephone network back towards the far-end talker. As a result the far-end talker, i.e. mobile station, hears an annoying echo of his/her own voice.
Conventional network echo cancellers typically employ adaptive digital filtering techniques. However, the filter used normally cannot precisely replicate the channel, thus resulting in some residual echo. A center-clipping echo suppressor is then used to eliminate the residual echo. The echo suppressor subjects the signal to a nonlinear function. Synthesized noise can be used to replace signal sections that were set to zero by the center-clipping echo suppressor to prevent the channel from sounding "dead".
Although the just mentioned echo cancellation approach is satisfactory for analog signals, this type of residual echo processing causes a problem in digital telephony. As mentioned previously, in a digital system vocoders are used to compress speech for transmission. Since vocoders are especially sensitive to nonlinear effects, center-clipping causes a degradation in voice quality. Furthermore, the noise replacement techniques used causes a perceptible variation in normal noise characteristics.
It is therefore an object of the present invention to provide a new and improved echo canceller capable of providing high dynamic echo cancellation for improved voice quality.
It is another object of the present invention to provide an echo canceller particularly suited for echo cancellation in the coupling of a digital communication system with an analog communication system.
It is yet another object of the present invention to provide an echo canceller with improved echo cancellation performance for cases where both parties are simultaneously talking.